1. Field of the Invention
This invention relates to a method for making an optical fiber preform which has a uniform dimension of a deposit along its leagthwise direction and is capable of being produced at a high speed. The invention also relates to an apparatus for carrying out the method.
2. Description of the Prior Art
In the initial stage of preparation of optical fiber preforms, there was used a method of covering a core glass with a glass tube clad as set out, for example, in Japanese Patent Publication No. 41-11071. Recent remarkable improvements in characteristics and precision and requirements for much larger-sized preforms have lead to methods disclosed, for example, in Japanese Laid-open Patent Application No. 49-84258. In the method of this laid-open application, a vapor phase starting material for glass is introduced into an oxyhydrogen flame of a burner to form a soot body (i.e. fine particles) of glass by hydrolysis with the flame. The soot is blown against the outer surfaces of a glass rod serving as a core, and the burner is reciprocated in axial directions in parallel to the glass rod to build up layers of the soot or fine particles of glass one by one along the radial directions to form a porous glass matrix. The matrix is heated to permit dehydration and vitrification thereby obtaining a transparent optical fiber preform.
Several improvements of this method have been proposed including a method, wherein the glass soot is continuously deposited in vertical directions, set forth, for example, in Japanese Laid-open Patent Application No. 55-116638, and a method set out, for example, in Japanese Laid-open Patent Application No. 57-183330, wherein while a core rod is reciprocated relative to a plurality of burners, different types of glass-forming materials are supplied as a porous glass matrix from the plural burners so that the composition of the glass-forming material changes every reciprocation, thereby obtaining a preform having a desired distribution of refractive indices along the radial directions. Aside from the above methods, there have also been proposed a method set out, for example, in Japanese Laid-open Patent Application No. 56-120528, in which while a core rod is rotated and moved along lengthwise directions, the rod is vibrated for the formation of glass particles. Moreover, Japanese Laid-open Patent Application No. 53-70449 has proposed a method of blowing glass fine particles against a core glass rod wherein a thin oxyhydrogen flame burner whose slit extends as having substantially the same width as the length of the core glass rod, or a burner array having a number of oxyhydrogen flame burners transversely arranged in line are used for the blowing without movement thereof. In addition, Japanese Laid-open Patent Application No. 64-9821 proposes a method wherein a porous glass matrix is prevented from crazing by controlling the feed of a gas from a plurality of burners or by controlling the distance between the level of the plural burners and the deposition level of soot, or by controlling the revolutions of a heat-resistant substrate to cause a deposition density of the soot to be changed along radial directions.
However, these known methods are not always satisfactory in many respects. More particularly, the method set out in the Japanese Laid-open Patent Application No. 49-84258 makes use of only one burner unit, with the attendant problem that the deposition rate of the soot or fine particles of glass is low and that where a long preform with a large diameter is prepared, the E quantity of heat becomes so short that the resultant deposited silica layer has low mechanical strength, thereby involving crazing occasion. Although the methods proposed in the Japanese Laid-open Patent Application Nos. 56-120528, 57-183330 and 58-9835 are advantageous in that the core layer and the clad layer can be formed by one step, the core and clad layers are so low in density that it is difficult to handle when an elongated preform with a large diameter is formed. These methods are also disadvantageous in that the fabrication apparatus becomes large in size and that when the core whose distribution of refractive index is not known is deposited with the clad layer, the resultant product may have characteristic properties which are outside the ranges of intended values.
The method of the Japanese Laid-open Patent Application No. 53-70449 is disadvantageous in that it cannot be guaranteed that the gas blown out from the slit of a burner can be maintained under the same conditions throughout the overall length of a glass rod serving as a core. This causes a deposit to be irregular at portions of individual burners or between adjacent burners. In fact, the resultant preform matrix has a poor accuracy with respect to the thickness of the deposit. With the method set out in the Japanese Laid-open Patent Application No. 64-9821, the deposition rate is high and a large-sized perform can be formed, but a burner is reciprocated at a given amplitude in the lengthwise directions, so that the burner is stopped at a given position and then moved to and stopped at another position, and returned to the given position. This reciprocation is invariably repeated between the two stop positions, thereby causing irregularities of deposition on the core. More particularly, the resultant preform has a deposit whose surfaces are irregular. In addition, aluminum used as a core material is undesirably doped in the silica layer as a metal impurity. Thus, this method cannot be utilized for the preparation of an optical fiber matrix.
For the preparation of a large-sized optical fiber preform at a high speed by use of a plurality of burners, Japanese Laid-open Patent Application No. 3-2288454 has proposed a method wherein a plurality of burners of the same design are located at equal intervals and the starting position of the reciprocating movement of the burners is changed every reciprocating cycle. This method has the advantage that the deposition rate remarkably increases, but with a serious problem that the amount of a deposit becomes non-uniform along the lengthwise direction.